Originally Posted by Mike@GTM

A return line generates a small, but measurable amount of evaporative emissions since the fuel traveling through the line will permeate through the line over time. Therefore, it would require more expensive material that prevents permeation known as SAE J30R7 rated hose. Even then, the longer the line, the more surface area that the fuel can permeate through. Therefore, it is ultimately less expensive to use a returnless fuel system to keep evaporative emissions low enough to pass EPA and CARB mandates. When it comes to OEM's, saving a few pennies per car translates to big savings over tens of thousands of cars.